Hapter 18 Ecology of Organisms and Populations


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Anatomical Responses – Many organisms respond to the environment by changing some aspect of their anatomy. Examples include growing a heavier coat of fur and hair. Plants are more finely tuned into this type of response. Plants can shift their energy allocations am ong various parts of their anatomy depending on changes in environmental conditions. Examples include growing towards light, devoting more energy to growing deeper roots in cases of drought, changing energy allocations between seed production and starch storage in the roots.

  • Behavioral ResponsesIn contrast to plants, animal can move to more favorable conditions in response to changing environmental conditions. This can be daily movements fomr shade to sunlight (reptiles). This can also include great long-distance movements with change in seasons – migration (mammals, birds, and fish). Behavioral responses also include building and maintaining shelters in preparation for incelement conditions.



    Population Ecology – Biologists define a population as a group of individuals of the same species living in the same area at the same time. Population ecology is the scientific study of how populations interact with the environment. There are many interesting questions that can be asked concerning populations and many population ecology studies have shed light on processes that contribute to population dynamics. This branch of biology has been instrumental in conservation and understanding evolution (remember, the population is the smallest unit on which evolution occurs). Population ecology has also helped us figure out how diseases spread.
    Here are a few things that population ecologists study:
    (1) Population density – the number of individuals of a species per unit are or volume. Determining how many individuals are in a given space helps to shed light on basic biology of the species. For instance, you can determine how much land is needed for the conservation of an endangered species until you do such basic research on population density. Ecologist often use Mark-Recapture Analysis which requires catching, marking, and re-capturing individuals of a population to determine an estimate of how many individuals live in that given amount of space. Various versions of this method use different formulas and sampling techniques but they all allow researchers to estimate population density.
    (2) Patterns of Dispersion – dispersion refers to the way in which individuals are spaced in a population. There are 3 general types of dispersion patterns: clumped, uniform, and random. A species will usually demonstrate 1 dispersion pattern and the dispersion pattern says a lot about the biology, ecology, natural history, conservation needs, and sometimes the evolution of a species.


    (3) Population Growth Models – the rates at which populations grow is a reflections of the reproduction biology of the respective species. Consider a single bacterium that can reproduce by dividing into 2 parts every 20 minutes. After 20 minutes there would be 2 bacteria, 4 after 40 minutes, and so on. If this continued for only 36 hours, there would be enough offspring of that original one bacterium to cover the entire earth 1 foot deep! At the other extreme, a pair of elephants may produce only 6 young over a period of 100 years. Still it would only take 750 years for a single pair of elephants to give rise to an eventual population of 19,000,000 elephants!
    So why doesn’t this happen? The answer is limited resources, disease, and so on

    Ecologists use several measures to calculate changes in population size. The growth of a population is equal to the number of births minus the number of deaths. Growth Rate is the change of in population size per time interval. There are 2 population Growth Models that we will look at here.
    (1) Exponential Growth Model – the rate of expansion of a population under ideal conditions in which the population multiplies by a constant factor during constant time intervals (generation time). The constant factor for bacteria is 2, because each parent cell divides to produce 2 daughter cells. The generation time for the bacteria in the graph below is 20 minutes. With each passing generation, the population size increases by an exponent of the number 2 – hence the name Exponential Growth Model. This Growth model occurs under ideal conditions and foe only short bursts of time. Eventually, limited resource become exhausted and/or disease spread more and more rapidly. In nature, we usually only see this type of growth when organisms are confronted with a new or under-exploited environment.



    (2) Logistic Growth Model – In nature, a population may grow exponentially for a short while but eventually, one or more environmental factors will limit its growth – at which the population will stops increasing or crashes. Environmental factors that restrict growth are referred to as population-limiting factors. Logistic Growth is characterized by carrying capacity, the highest number of individuals which an environment can support; remember resources are limited.

    Notice that with Logistic Growth, a population tends to grow rapidly when resources are abundant but growth starts to rapidly slow and even come to a halt as resources become depleted. Resources can include food, water, light, oxygen, mates, shelter, breeding sites, nesting space, and so on.

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